1. Field of the Invention
This invention relates to an evacuation apparatus useful for high vacuum evacuation systems which require cleanliness and high airtightness, such as, semiconductor manufacturing plants, evacuation equipment of radioactive gases occurring in nuclear power plants and particle accelerators, medical facilities and space engineering facilities.
2. Description of the Prior Art
In constructing an evacuation apparatus used in evacuation systems for semiconductor manufacturing plants and the like, it is customary to arrange a roughing pump on the atmospheric side of the system and a high vacuum pump on the vacuum side of the system. It is thus common to use two pumps in an evacuation system in which the operation is carried out under high vacuum. The conventional combination of two pumps generally employed for this purpose is one in which an oil-sealed rotary pump is installed on the atmospheric side of the system and a mechanical booster or a turbomolecular pump is used on the vacuum side of the system, depending on the operation pressures.
In the prior art, however, there still exist unsolved technical problems as described below. backdiff
In producing a high vacuum, an oil-sealed rotary pump used as a roughing pump causes back-diffusion of the working oil to the apparatus being evacuated because the pump casing is filled with the oil, thus resulting in a reduction of the yield of satisfactory products produced in the apparatus under vacuum. Further, a process gas can react with the working oil to promote the degradation of the oil, or to cause fine particulates of reaction products produced by a reaction between the material processed under vacuum such as SiO.sub.2 wafer and the process gas to migrate in the pump and trapped by the oil wetting the inside of the pump, and cause bad effects, such as damage of the pump. Since such fine particulates are dry, they would not be so trapped without such an oil, and would be delivered out of the pump. These problems have been pointed out for a long time by persons interested in semiconductor manufacturing plants and the like.
Even in the case of using a pump other than an oil-sealed rotary pump, as disclosed in Japanese Patent Laid-Open No. 291479/1987, by way of example, if the pump is a conventional rotary or reciprocating pump, it also involves the above-described problems to a greater or lesser degree because these types of pumps do not use a structure in which the oil-lubricated section and the vacuum side are completely isolated from each other, as is the case with the pump (I) of the invention as described below.